Allow input to be passed in though stdin

[chrishoage]

eslint_d allows for input though stdin, this allows for background mode in sublime linter to work (as SublimeLinter will pass in the current buffer of the file)

With out the ability to use stdin background mode always uses the saved version of the file, not the version in the buffer.

[jo-sm]

I updated stylelint_d to support input via stdin using the --stdin flag. If you want to use it standalone, you'll need to pass in either the config file location, or the file location, with the --config or --file flags respectively.